You'll probably also go through 3-4(?) pairs of running shoes, which are probably around $75-100 each, for a total of $300.
You might want some extra work-out clothes. Most of these could be cheap, but you'll probably want something fairly fancy for the race day itself--running 26 miles in a heavy cotton shirt sounds miserable. A few hundred dollars doesn't seem crazy, depending on what you already own. You could economize here--or you could go crazy.
You might want some kind of coaching, or possibly a gym membership to mix up your training (e.g., swim when hurt). This seems to depend on a lot on where you live, but $50/month doesn't seem nuts for NYC. Over a year, that's $600.
That should put you in the ballpark.